Method for Starting a Hydraulic Turbine

ABSTRACT

The invention concerns a method for coupling to the grid a hydraulic unit having a synchronous generator, a runner, and wicket gates, the method comprises:
         a) a step of increasing the flow of water into the runner from a time t 0  to a time t 1  so that the rotation frequency of the rotor of the synchronous generator is, at time t 1  equal to the frequency of the grid;   b) a step of closing the circuit breaker at time t 1 ,   step a) further comprises a sub-step a1) executed from a time t 2  to time t 1 , wherein the flow of water is adjusted so that, at time t 1 , the phase of the synchronous generator is aligned with the grid phase.

TECHNICAL FIELD

The invention relates to a method for coupling a hydraulic turbine tothe grid. In particular, the invention relates to a method for couplingto the grid a hydraulic turbine which comprises a synchronous generatorwhose rotor is driven in rotation by a turbine. The present invention isfurther intended to propose a method for coupling a hydraulic turbine tothe grid in faster way than prior art methods.

PRIOR ART

In order to ensure a grid stability and/or to prevent a networkblackout, a grid balancing between the electrical power production andthe electrical power consumption must be achieved.

Hydroelectric power plants have an electrical power reserve, via waterreserves contained in a reservoir that can be provided upon demand bystarting a hydraulic turbine in order to compensate any variation of theconsumption and/or the production of the electrical power.

To this regard, the time response for providing such an electrical powerreserve is thus a critical factor, on the basis of which the electricityproducer can expect a more or less advantageous remuneration.

A known prior art method for starting a hydraulic turbine having asynchronous generator connected to a grid via a circuit breaker, arunner to drive the synchronous generator, and guide vanes for adjustinga flow of water running into the runner, comprises the following steps:

a) Opening the guide vanes (wicket gates) to pass the flow of water inthe runner and bring the runner speed up to a rated speed;

b) Circulating an excitation current in the rotor windings in order toenergize the synchronous generator;

c) Adjusting the guide vanes to stabilize the runner speed at a valueclose to a nominal speed which corresponds to the frequency of the grid;

d) Aligning the phases of the grid with the phases of the synchronousgenerator;

e) Closing the circuit breaker to connect the stator winding to thegrid.

However, this method is not satisfactory.

Indeed, the minimum time response for starting a hydraulic turbine isabout 90 seconds, which is not compatible with the maximum efficiencythat could be expected in terms of revenues.

For example, steps c) and d), which taken together form a more generalstep called “synchronization step”, generally take 20 s to 60 s. Thecontrol of the unit speed is indeed very difficult due to both thetransient behavior of the hydraulic circuit and the difficulty tocontrol slightly the water flow with the distributor.

Therefore, to keep the time response of the hydraulic unit starting aslow as possible, it has been proposed to maintain said unit synchronizedto the grid at very low power.

However, this solution is also not satisfactory.

Indeed, when synchronized at very low or zero power, the hydraulic unitconsumes water reserves while operating at very low efficiency.

Besides, turbines and synchronous generators, which are generally notdesigned to operate at low power, then undergo abnormal wear.

It is possible to operate at higher power but such additional power,even if not needed, has to be managed by the grid operator and is soldat a very low price.

Alternatively, the hydraulic unit can be kept synchronized to the gridin a dewatered mode.

However, the alternative is also not satisfactory.

Indeed, the dewatered mode requires specific investments for flushingthe water from the hydraulic turbine, which increase the cost of theinstallation.

Furthermore, in dewatered mode, the hydraulic unit consumes electricalpower form the grid.

It is therefore an object of the invention to propose a method forcoupling a hydraulic unit to the grid in a faster way than known priorart method.

It is also an object of the invention to propose a method for couplingthe hydraulic turbine to the grid that does not require any additionalinvestments.

It is also an object of the invention to propose a method for couplingthe hydraulic unit to the grid that does not consume neither waterreserves nor grid electrical power.

SUMMARY OF THE INVENTION

The aforementioned objects are, at least partly, achieved by a methodfor controlling, advantageously starting, a hydraulic unit having asynchronous generator intended to be connected to a grid via a circuitbreaker, a hydraulic turbine provided with a runner mechanically coupledto the rotor of the synchronous generator via a shaft line to drive thesynchronous generator, and means for adjusting a torque to the shaftline, the method comprises:

a) a step of adjusting, advantageously increasing, the flow of waterinto the runner from a time t₀ to a time t₁;

b) a step of closing the circuit breaker at time t₁, t₁ being a time forwhich the rotation frequency of the rotor of the synchronous generator,called generator frequency, is equal to the frequency of the grid withina first tolerance interval and the grid phase and the synchronousgenerator phase are aligned within a second tolerance interval,

the step a) comprises a sub-step a1) executed from a time t₂ to time t₁,wherein an adjustment torque, calculated with a controller calledadjustment controller, is applied to the shaft line so that, at a timet₃, for which the generator frequency enters for the first time withinthe first tolerance interval, the difference between the synchronousgenerator phase and the grid phase, is equal to a predetermined value Awithin a third tolerance interval, time t₁ being the nearest time withrespect to time t₃ for which the grid phase and the synchronousgenerator phase are aligned within the second tolerance interval.

The method may advantageously be used for starting the hydraulicturbine. The method may also be used when a reversible turbine changesfrom a pumping mode to a turbine mode. The method may also be used tocouple the turbine back to the grid after a default causes adisconnection of the turbine from the grid.

According to one embodiment, the predetermined value A is in the range[0°, 270°], preferably [0°, 180°], and more preferably [0°, 45°] if thefrequency of the grid is above the frequency of the generator, or in therange [−270°, 0°], preferably [−180°, 0°], and more preferably [−45°,0°] if the frequency of the grid is below the frequency of thegenerator.

According to one embodiment, time t₂ is a time for which the rotationfrequency of the rotor, called intermediate rotation frequency, is below95%, preferably 80%, more preferably below 70%, of the grid frequency.

According to one embodiment, the torque is adjusted via adjustment ofthe flow of water.

According to one embodiment, the flow of water is adjusted with wicketgates.

According to one embodiment, the adjustment torque comprises anadjustment of the flow of water running into the runner via an actuator,called first actuator, the first actuator being controlled by acontroller, called adjustment controller, and wherein during executionof sub-step a1), the adjustment controller calculates, advantageously inreal time, the needed flow of water running into the runner.

According to one embodiment, the adjustment controller get signalsprovided by sensors adapted for measuring the generator frequency andthe generator phase.

According to one embodiment, the adjustment controller is furtherprovided with a hydrodynamic based model for the calculation of theneeded flow of water running into the runner.

According to one embodiment, the hydrodynamic based model is ahydrodynamic analytical model of the hydraulic turbine.

According to one embodiment, the adjustment controller controls a secondactuator adapted for applying an electrical torque on the rotor of thesynchronous generator during sub-step a1).

According to one embodiment, the second actuator comprises a variablefrequency drive. The variable frequency drive can be a static frequencyconverter or a voltage source inverter.

According to one embodiment, the second actuator uses a feedback loopfor controlling the electrical torque during sub-step a1).

According to one embodiment, the adjustment controller calculates atheoretical trajectory of frequency rising of the rotor.

According to one embodiment, the second actuator controls the electricaltorque in real time so that the frequency rising of the rotor sticks tothe theoretical trajectory.

According to one embodiment, the second actuator is powered by an AC/DCconversion unit coupled with a battery, or a thyristor bridge connectedto the grid.

The invention also concerns a computer program for implementing themethod of coupling a hydraulic unit to the grid according to the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages shall appear in the followingdescription of embodiments of the method for coupling a hydraulicturbine to the grid according to the invention, given by way ofnon-limiting examples, in reference to the annexed drawings wherein:

FIG. 1 is a schematic representation of a hydraulic unit according to afirst embodiment of the invention involving a controller which controlstwo actuators and a feedback loop;

FIG. 2 is a schematic representation of a hydraulic unit according to afirst variant of a second embodiment of the invention involving avariable frequency drive fed by the power of the grid;

FIG. 3 is another schematic representation of a hydraulic unit accordingto a second variant of the second embodiment of the invention involvinga variable frequency drive fed by a battery.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention proposes a method for coupling a hydraulic unit tothe grid in a faster way than known prior art methods. In particular,and contrary to known prior art methods, the method of the presentinvention proposes to execute simultaneously the generator frequencystabilization and the alignment of the generator phase with the gridphase.

FIG. 1 depicts an overall architecture of a hydraulic unit according tothe present invention.

The hydraulic unit 1 comprises a synchronous generator 2 connected to agrid 3 via a circuit breaker 4 (displayed in FIGS. 2 and 3).

It is understood that, without it being necessary to specify, thesynchronous generator comprises a rotor arranged to rotate inside astator whose windings are connected to the grid via the circuit breaker.

Furthermore, it is also understood that, without it being necessary tospecify, the grid is circulated by an alternative electrical current offrequency called grid frequency; and phase called grid phase.

The present invention, for sake of clarity, is described in the contextof a single phase synchronous generator, and can be generalized to amultiple phase (for example a three phases) synchronous generator.

The hydraulic unit 1 further comprises a hydraulic turbine 5 providedwith a runner 6 mechanically coupled to the rotor of the synchronousgenerator 2 via a shaft line 9, so that when rotating, the runner 6drives into rotation the rotor at the same rotation speed. The rotationspeed of the rotor directly determines the frequency of the electricalvoltage delivered by the synchronous generator via the number of polesof the rotor.

Besides, the hydraulic unit 1 also comprises means for adjusting atorque on the shaft line.

For example, the adjusting means can comprise wicket gates.

The hydraulic unit 1 may also comprise, upstream of the wicket gates, amain inlet valve.

The method for coupling the hydraulic unit to the grid according to thepresent invention comprises a step a) of increasing the flow of waterinto the runner from a time t₀ to a time t₁. At time t₁, the circuitbreaker may be closed to connect the generator to the grid, t₁ being atime for which 2 conditions are met:

1) the rotation frequency of the rotor of the synchronous generator,called generator frequency, is equal to the grid frequency within afirst tolerance interval, and

2) the grid phase and the synchronous generator phase are aligned withina second tolerance interval.

The first tolerance interval is for example an interval of +/−0.5%,preferably 0.2%, centered on the grid frequency.

The second tolerance interval is for example an interval of +/−20°,+/−10°, preferably +/−5°, and more preferably +/−2°, centered on thegrid phase.

At a certain rotation frequency, the windings of the rotor can becirculated by an excitation current for energizing the synchronousgenerator.

The step a) further comprises a sub-step a1) executed from a time t₂ totime t₁, wherein an adjustment torque is applied to the shaft line sothat, at a time t₃, for which the generator frequency enters for thefirst time within the first tolerance interval, the difference betweenthe synchronous generator phase and the grid phase, is equal to apredetermined value A within a third tolerance interval.

It is understood that, without it being necessary to specify, that timet₂ is comprised in the [t₀ t₁] range.

It is also understood that, without it being necessary to specify, thattime t₃ is comprised in the [t₂, t₁] range.

Advantageously, if the frequency of the grid is above the frequency ofthe generator, A is in the range [0°, 270°], preferably [0°, 180°], andmore preferably [0°, 45°].

Advantageously, if the frequency of the grid is below the frequency ofthe generator, A is in the range [−270°, 0°], preferably [−180°, 0°],and more preferably [−45°, 0°].

The third tolerance interval is for example an interval of +/−135°,+/−90°, preferably +/−22.5°, and more preferably +/−10°, centered on thevalue of A.

Time t₂ is, for example, a time for which the rotation frequency of therotor, called intermediate rotation frequency, is below 95%, preferably80%, more preferably below 70%, of the grid frequency.

As way of example, the method of coupling the hydraulic turbine to thegrid according to the present invention is described below with apredetermined value A equal to zero. This example represents an idealcase.

The hydraulic turbine can comprise a controller, called adjustmentcontroller 7, adapted to control the means for adjusting the torque tothe shaft line.

In particular, during execution of sub-step a1), the adjustmentcontroller 7 calculates, advantageously in real time, the adjustmenttorque to be applied to the shaft line 9.

To do this, the adjustment controller 7 can receive information, likethe difference between the generator frequency and the grid frequency orthe difference between the generator phase and the grid phase.

Frequencies and phases may be measured by sensors. Considering thosedifferences, the adjustment controller 7 can estimate or calculate, viaan algorithm, a torque to be applied to the shaft line to adjust thefrequency rise up of the rotor so that, at time t₃, the differencebetween the phase of the synchronous generator and the grid phase isequal to zero. In this example, t₁ occurs at the same time as t₃: thefrequency of the grid and the synchronous generator are within a firsttolerance interval of each other while the phases of the grid and of thesynchronous generator are aligned within the second tolerance interval.The circuit breaker is closed in order to feed electrical current to thegrid.

This ideal case may only be achieved if there is no uncertainty due tothe controller 7 and/or of the actuator 11.

If there is some uncertainty due to the controller 7 and/or of theactuator 11, the difference between the phase of the synchronousgenerator and the phase of the grid may not be strictly equal to zero attime t₃. The difference is indeed comprised within the third toleranceinterval.

Knowing the third tolerance interval and if the frequency of the grid isabove the frequency of the generator, it may be interesting for thecontroller to calculate the torque to be applied to the shaft line sothat at time t₃, the difference between the phase of the generator andthose of the grid is always positive despite the uncertainties. For thisreason, the value of A is advantageously set to a positive value if thefrequency of the grid is above the frequency of the generator.

Alternatively, if the frequency of the grid is below the frequency ofthe generator, the value of A is advantageously set to a negative value.

The optimal value of A is half the length of the third interval, thesign of A depending on the relative position of the generator frequencyand those of the grid.

The consequence of a value A not equal to zero is that t₁ occurs acertain time after t₃, causing a performance degradation of the timeneed to couple the hydraulic unit to the grid.

A value A near zero allows a short coupling time. The performances ofthe invention depends then directly on the third tolerance interval.That is one of the reasons why the accuracy of the controller or thoseof the actuator that applies the torque on the shaft line must be muchhigher than those usually used to start hydraulic units.

In one embodiment (FIG. 1), the adjustment torque applied to the shaftline comprises an adjustment of the flow of water running into therunner. In particular, the adjustment of the flow of water comprises theimplementation of an algorithm based on an advanced control solutioncalled model predictive control. This model is integrated multiple timesover a horizon of typically 10 s during one sample time of the regulator(typically 10 ms) in order to find the best speed/phase trajectory andthe corresponding command for the actuator(s). This control strategy iscomputational resource intensive and it allows to take into accountprecisely the dynamic behavior of the hydraulic circuit, such as thewater hammer in the penstock and to master the speed/phase behavior.Further details regarding the modeling of the hydro generator are givenin reference [1] cited at the end of the description.

The adjustment controller 7, via the algorithm, can therefore estimatethe optimal opening set point of the wicket gates for achieving sub-stepa1). Said estimation of the optimal opening is therefore communicated toan actuator, called first actuator 10, that controls the opening of thewicket gates so that the most appropriate hydraulic torque is applied tothe shaft line.

The estimation of the optimal opening set point is advantageouslycarried out in real time, for example by continuously measuring thefrequency and the phase of the synchronous generator, and by the use ofan extended Kalman filter 12.

The inventors have noticed that, when executing the method according tothe invention, the delay between time t₃ and time t₁ and thepredetermined value A are achieved in a repeatable manner from oneexecution to the other.

In another embodiment (FIGS. 2 and 3), the adjustment controller 7 canimpose set points to a second actuator 11. The first actuator 10controls the opening of the wicket gates, whereas the second actuator 11can comprise a variable frequency drive coupled to the stator of thesynchronous generator.

The variable frequency drive is a power electronics device that providesan electrical torque to the shaft line via the stator windings. Thepower electronic device can be a static frequency converter or a voltagesource inverter.

During sub-step a1), the adjustment controller 7 computes a theoreticaltrajectory of frequency of the generator based on the information offrequency and phase of the synchronous generator, so that at time t₁ thesynchronous generator is synchronized and aligned with the grid. Oncethe theoretical trajectory has been calculated, the adjustmentcontroller 7 adjusts therefore the set points of the first and thesecond actuators so that the frequency trajectory of the synchronousgenerator sticks to the theoretical trajectory. This control isadvantageously carried out with a feedback loop control to prevent orminimize any deviation from the theoretical trajectory.

The second actuator 11 can comprise an AC/DC conversion unit coupledwith a battery 8.

The method according to the present invention further comprises a stepb), executed at time t₁, of closing the circuit breaker 4 to fed thegrid with the current produced by the synchronous generator.

The electrical actuator 11, after closing the circuit breaker, can bedisconnected from the hydraulic unit by opening a switch 7 a.

In another embodiment, the adjustment controller can impose anappropriate setpoint to both first actuator 10 and second actuator 11.

Therefore, according to the present invention, it is possible to executeboth phase synchronization and phase alignment within a single step inorder to reduce the time needed to couple the hydraulic unit to thegrid.

For example, the inventors have numerically demonstrated that thestarting of a hydraulic unit can be reduced from 90 seconds to 60seconds.

Furthermore, the method according to the present invention does notrequire the implementation of any additional equipment, and can thus beadapted to existing hydroelectric power plants.

-   [1] Hugo Mesnage et al., «Constrained model based control for    minimum-time start of hydraulic turbines», 28^(th) AIRH, Grenoble    2016.

1. Method for controlling, advantageously coupling to the grid, ahydraulic unit having a synchronous generator intended to be connectedto a grid via a circuit breaker, a hydraulic turbine provided with arunner mechanically coupled to the rotor of the synchronous generatorvia a shaft line to drive the synchronous generator, and means foradjusting a torque to the shaft line, the method comprises: a) a step ofadjusting, advantageously increasing, the flow of water into the runnerfrom a time t₀ to a time t₁; b) a step of closing the circuit breaker attime t₁, t₁ being a time for which the rotation frequency of the rotorof the synchronous generator, called generator frequency, is equal tothe frequency of the grid within a first tolerance interval and the gridphase and the synchronous generator phase are aligned within a secondtolerance interval, the method being characterized in that the step a)comprises a sub-step a1) executed from a time t₂ to time t₁, wherein anadjustment torque, calculated with a controller called adjustmentcontroller, is applied to the shaft line so that, at a time t₃, forwhich the generator frequency enters for the first time within the firsttolerance interval, the difference between the synchronous generatorphase and the grid phase, is equal to a predetermined value A within athird tolerance interval, time t₁ being the nearest time with respect totime t₃ for which the grid phase and the synchronous generator phase arealigned within the second tolerance interval.
 2. Method according toclaim 1, wherein the predetermined value A is in the range [0°, 270°] ifthe frequency of the grid is above the frequency of the generator, or inthe range [−270°, 0°] if the frequency of the grid is below thefrequency of the generator.
 3. Method according to claim 1, wherein timet₂ is a time for which the rotation frequency of the rotor, calledintermediate rotation frequency, is below 95% of the grid frequency. 4.Method according to claim 1, wherein the means for adjusting a torquecomprise wicket gates.
 5. Method according to claim 1, wherein theadjustment torque comprises an adjustment of the flow of water runninginto the runner via an actuator, called first actuator, the firstactuator being controlled by the adjustment controller, and whereinduring execution of sub-step a1), the adjustment controller calculates,advantageously in real time, the needed flow of water running into therunner.
 6. Method according to claim 5, wherein the adjustmentcontroller calculates in real time, the needed flow of water runninginto the runner.
 7. Method according to claim 5, wherein the adjustmentcontroller is communicating with sensors adapted for measuring thegenerator frequency and the generator phase.
 8. Method according toclaim 6, wherein the adjustment controller is communicating with sensorsadapted for measuring the generator frequency and the generator phase.9. Method according to claim 5, wherein the adjustment controller isfurther provided with a hydrodynamic based model for the calculation ofthe needed flow of water running into the runner.
 10. Method accordingto claim 7, wherein the hydrodynamic based model is a hydrodynamicanalytical model of the hydraulic unit and the feedback loop contains astate estimator.
 11. Method according to claim 10, wherein the stateestimator of the feedback loop is based on an extended Kalman filter.12. Method according to claim 5, wherein the adjustment controllercontrols a second actuator adapted for applying an electrical torque onthe rotor of the synchronous generator during sub-step a1).
 13. Methodaccording to claim 12, wherein the adjustment controller calculates atheoretical trajectory of frequency rising of the rotor.
 14. Methodaccording to claim 13, wherein the second actuator controls theelectrical torque in real time so that the frequency rising of the rotorsticks to the theoretical trajectory.
 15. Method according to claim 12,wherein the second actuator comprises a variable frequency drive. 16.Method according to claim 12, wherein the second actuator uses afeedback loop for controlling the electrical torque during sub-step a1).17. Method according to claim 12, wherein the second actuator is poweredby an AC/DC conversion unit coupled with a battery, or a thyristorbridge connected to the grid.
 18. Computer program for implementing themethod according to claim 1.